Field of the Invention
The present invention relates to an illumination arrangement, in particular an illumination arrangement for an endoscope, a beam combination device and a method for coupling at least three input light beams into an optical waveguide, in particular into an optical waveguide of an endoscope.
Description of the Background Art
Endoscopic examination techniques have gained acceptance in a multiplicity of fields of application appertaining to medicine and animal medicine, but also in specific technical fields of application. In this case, an endoscope having an elongated shank with an imaging optical system is inserted into a cavity in order there to record an image of an object field and to transmit it to somewhere outside the body and thus to provide it for viewing and/or evaluation. The shank can be embodied such that it is rigid, semi-rigid or flexible. In order to illuminate the object field, the illumination light generated in the proximal region of the endoscope is usually guided to the distal end of the shank of the endoscope with the aid of optical fibers. Since the permissible external diameter of the shank of the endoscope is narrowly limited in many applications, the use of optical fibers having the smallest possible external diameter is advantageous.
For the endoscopic illumination, the illumination light generated by a light source is coupled into the optical fibers. Precisely in the case of very thin endoscopes it is desirable to use very thin optical fibers having a thickness of just a few 100 μm; in addition, said optical fibers have a high degree of flexibility, and so such illumination fibers can also be used in flexible endoscopes. However, conventional lamp systems such as xenon lamps or LEDs, for example, can be coupled into individual optical fibers having diameters of a few 100 μm only with high losses of illumination intensity. In principle, laser diodes make it possible to generate sufficiently bright illumination light which can even be coupled into very thin optical fibers with relatively low losses. Since laser diodes generate monochromatic light, for the endoscopic observation it is necessary to use a plurality of laser diodes which each generate light having a different wavelength. In particular, a combination of the three fundamental colors red, green and blue affords the possibility of generating white light. However, this necessitates a combination of the red, green and blue laser diode radiation generated by the respective laser diodes and coupling into a thin optical fiber.
Conventional RGB laser systems typically achieve a combination of the radiation of the red, green and blue laser diodes by virtue of the fact that the latter are fiber-coupled and the fibers of the individual laser diodes are combined in a common ferrule. This is followed by a further optical fiber encompassing the combined cross section of the three optical fibers of the laser diodes. As a result, this optical fiber that forwards the combined red-green-blue laser radiation necessarily has a larger diameter than the individual fibers connected to the respective laser diodes; such a combination is therefore unsuitable for particularly thin optical fibers. A combination of the red-green-blue laser diode radiation in a thin fiber can be achieved, in principle, by means of dichroic mirrors and lens systems. However, such arrangements have a high degree of adjustment sensitivity.
WO 2010/127694 A1 discloses an optical microprojection system in which three laser light sources for generating red, green and blue light and a beam combination device are provided for colored projection. The beam combination device is composed of optical components having coatings for wavelength-dependent reflection and transmission. The three laser light sources each have a collimator lens. The output beam, constituting a superimposition of the three light sources, is directed to the projection surface via a beam splitter cube, two quarter-wave plates and two movable micromirrors. A beam combination for coupling into an optical fiber is not provided.
In accordance with US 2002/0101634 A1 it is known that in a polarization beam combination device (Polarization Beam Combiner, PBC) used in optical communications technology, a plurality of dielectric layers on a diagonal surface between two prisms are used to combine with one another two beams that are incident at right angles to one another. A birefringent crystal can be used to combine two orthogonally polarized beams. US 2002/0101634 A1 proposes arranging a birefringent crystal having a surface inclined by approximately 3° to 30° between two GRIN lenses. Two entering beams, the polarization directions of which are directed in accordance with the axes of the birefringent crystal, are collimated by a GRIN lens and combined by the birefringent crystal into a single beam that is collimated by the second GRIN lens and coupled into an output fiber. The combination of more than two beams is not possible in this case.